At first, it was the 8,000-square-foot front lawn, which was found to be contaminated by arsenic. Then, more contamination was found and fence after fence popped up: in the large courtyards where students lounge at lunch, around planters around the school, at a practice field behind the school.

But all those fences should finally come down this summer, as the high school’s $4.2 million arsenic cleanup at long last comes to a close.

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“It’s been a real aggravation for students, and it’s taken a lot longer than anticipated,” said Liz McManus, assistant superintendent for the San Mateo Union High School District. “Everybody’s really pleased to have everything almost done.”

Since 2003, environmental testing has found areas of the campus contaminated by arsenic, as well as lead and polychlorinated biphenyls, all of which can cause health problems.

The PCB contamination was found near the former site of a large transformer, believed to have been a part of the school’s electrical system until the 1950s, said Todd Lee, the project manager for the cleanup. Lead was found in the soil right around the buildings and is thought to have come from lead-based paint, he said.

The origins of the arsenic are hazier, perhaps coming from an arsenic-based pesticide, or maybe from a former use of the land by a tannery, he said.

The school district initially considered solving the problem by installing a permanent 5-foot fence around the front lawn at a cost of about $250,000, McManus said. However, it was later decided that a cleanup was a better strategy, she said.

As of this week, nearly all of the contaminated soil has been carted away. The district’s board is expected to approve a $756,000 contract to replace the landscaping at the formerly contaminated areas.

McManus said all closed areas should be reopened by the start of the next school year, which will come as a relief to students and teachers who have been living with noise, dust and the inconvenience of learning in a construction zone during the last year, she said.

“There will be new grass in the area, new landscaping, areas for students to meet and greet and friends can gather,” she said. “It’ll be nice — a pleasant upgrade to what they’ve had in the past.”